Stimulation of the Bainite Transformation Scenario by an External Magnetic Field

Introduction. It makes practical sense to change the properties of steels with a bainite structure, as with bainite transformation under the influence of a magnetic field, it is possible to improve the ductility of the steel while maintaining or even increasing its strength. Scientific research in this area has focused on the influence of the magnetic field on thermodynamics and on the change in the phase transformation scenario. However, there is no detailed description in open sources of the effect of a magnetic field on the structure and properties of the products of intermediate bainite transformation. The aim of the work is to study the peculiarities of the influence of an external magnetic field on the scenario and kinetics of phase transformation of steel.

Materials and Methods. The study was conducted using samples made of 65G steel. Their chemical composition was monitored using a Magellan Q8 optical emission spectrometer. Heat treatment (resistive heating) was carried out in an IMASH 20–75 installation for high-temperature research. The heating temperature was approximately 1000 degrees 1000°C, and the holding time was 10 minutes. The sample was cooled down using water-cooled electrical contacts. An external magnetic field with a strength of 400 kA/m and 800 kA/m was created by an electromagnet integrated into the vacuum chamber of the installation.

Results. The experiments confirmed the potential for altering the transformation pathway from pearlite into bainite in the presence of an external magnetic field of up to 1 MA/m. Images of the microstructure and surface relief of samples after cooling in a magnetic field were obtained. Kinetic changes and dependencies of the volumetric transformation rates on the duration of isothermal exposure were analyzed. It has been found that exposure to a constant magnetic field of 1.6 MA/m increased the volumetric transformation rate by 1.808 times (for 65G steel) and by 1.687 times (for 45H steel).

Discussion and Conclusion. The results of observations of changes in the surface relief during cooling in the absence of a magnetic field, and in magnetic fields of various strengths, were recorded. This has allowed us to draw the conclusion that the external magnetic field stimulates the bainitic transformation instead of the original pearlitic one. Microstructural changes can be explained by the influence of the magnetic field on the initial phase magnetic state.

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